1. Field of the Invention
This invention relates generally to a micromotor with built-in cooling medium pipes and more particularly to improvements in a small-size, high power micromotor used in a dental handpiece and other medical appliances of the type wherein a stator of the micromotor structurally contains a permanent magnet and a magnetic yoke encircling the outer circumference of the permanent magnet.
2. Prior Art
In a dental handpiece, for example, a pipe for feeding a cooling medium (mainly water) for preventing heat generated during tooth cutting is used in combination with a handpiece and a supply of water during tooth cutting from the front end of the pipe to a point of the region to be cut is a minimum requirement. Heretofore, handpieces have been classified into a type having such a cooling medium pipe mounted outside thereof and a type including the pipe inside thereof
Recently, in order to improve the cutting property of a handpiece, various attempts have been made to improve also the micromotor used in the handpiece. For example, the material used in the micromotor has been changed from a conventional alnicoic material to a material of high energy density, such as rare earth cobaltic material. In this case, a stator not made of alnicoic material but a stator having outer cicumference of a permanent magnet made of rare earth cobaltic material and encircled with a yoke has been proposed so as to further strengthen the effective magnetic flux and to promote miniaturization of a motor. The device of the invention relates to a structure of such a type of micromotor which makes it possible to incorporate a micromotor cooling medium pipe in the micromotor without changing the performance of the motor nor increasing the size of the motor. A description will be given below of a structure of a conventional dental micromotor having a cooling medium pipe therein and of the disadvantages of the structure.
(i) A nonmagnetic body portion encircling a stator is disposed between a motor housing and the stator; and an area for containing the cooling medium pipe therein is provided in a nonmagnetic body portion.
(ii) Two pieces of permanent magnet made of alnicoic material and shaped arcuate in section are disposed in opposed relation and two yokes corresponding to a sector area between two magnet pieces are interposed between such two magnet pieces so as to form a cylindrical stator as a whole and a pipe for a cooling medium is disposed substantially in the middle of the yokes.
The structure thus obtained prevents the pipe from producing an adverse effect on effective magnetic flux and accordingly, the structure offers the advantage that the motor need not be increased in size. However, the structure above cannot be applied to a motor in which are used a magnet of rare earth cobaltic material that has recently come to be used instead of alnicoic material and a yoke encircling the outer cicumference of the magnet.